In recent years, energy saving is required from a point of view of preventing global warming, and further reduction in power consumption is required in fields such as a motor of an air conditioner and a main motor of an electric vehicle. These motors are often used in high rotation, and therefore, improvement in core loss at a region of 400 Hz to 800 Hz being higher frequency than 50 Hz to 60 Hz being a conventional commercial frequency is required for a non-oriented electrical steel sheet (hereinafter, there is a case when it is described as a “steel sheet”) to be a motor material.
As a measure to improve the core loss at the high-frequency region of the non-oriented electrical steel sheet, it is generally performed to increase electrical resistance by increasing contents of Si and Al as described in, for example, Patent Literature 1. Note that recently, there is a case when an alloy raw material of Si and Al whose Ti content is high is used as a cheap alloy raw material to reduce cost.
According to the increase of the contents of Si and Al, Ti having high affinity with these elements is inevitably contained in the alloy raw material, and therefore, Ti is inevitably mixed into the steel sheet. When Ti in the steel sheet is 0.001 mass % or more, a number of fine Ti inclusions whose diameters are approximately several dozen nm such as TiN, TiS, TiC are generated in the steel sheet. The fine Ti inclusions in the steel sheet may disturb a growth of crystal grains at an annealing time of the steel sheet, and deteriorates magnetic properties.
Accordingly, it is necessary to reduce the Ti inclusions in the steel sheet as much as possible. One of measures of the above is to use the alloy raw material whose Ti content being an impurity is small. However, there is a problem to incur a cost increase of the alloy raw material if this measure is taken. Besides, it is also one of the measures to reduce the Ti inclusions by decreasing N, S and C in the steel sheet, and it is possible with current technology to enough decrease S and C by a vacuum degassing treatment and so on. However, the treatment for a long time is necessary to decrease S and C in the steel sheet, and productivity is thereby lowered. Besides, it is also conceivable to enhance sealing of a refining vessel not to mix N into molten steel, but it incurs the cost increase caused by the enhancement of the sealing, and further, there is a problem that the mixture of N into the molten steel is inevitable even if the treatment as stated above is performed.